The invention relates to a gearbox which is switchable under load. The invention includes at least two parallel shafts which achieve different transmission stages. Each shaft carries gears which are permanently engaged with each other in pairs. Each one of the gears may be connected to its associated shaft via a friction coupling switchable by an axially effective operating assembly.
Conventional automatic gearboxes which have planetary gears and are used as four-gear gearboxes with directly driven highest gears, while eliminating converters, meet the highest demands regarding comfort and economy. However, these systems are technically complicated and expensive.
Double clutch gearboxes (Porsche PDK gearbox) which, in principle, are designed like conventional manual gearboxes, are also able to switch without interrupting the traction force. The double clutch and the shaft design include a hollow shaft and inner shaft which increase production costs.
Automatic mechanical gearboxes whose design corresponds to the design of conventional manual gearboxes with only an automated coupling operation are disadvantageous because of the interruption of the traction force. In addition, these systems are expensive relative to the performance which they offer
A review of the different gearbox designs is given in "1. Aachener Kolloquium Fahrzeug- und Motorentechnik" (1st Aachen Colloquium on vehicle and engine technology), October, 1987, conference papers, page 385 ff.
GB 1 582 237 proposes an automatic gearbox in which the switching sleeves are replaced by hydraulically operated friction couplings. The supply of the pressurized fluid via axial shaft bores and the control thereof is too complicated for cost effective production purposes and interference-free operation.
The subsequently published, former patent applications DE 39 28 816 A1 and DE 40 07 506 A1 describe a gearbox of the initially mentioned type. The gearbox is characterised by the operating assembly of its friction couplings. The couplings include a first adjusting ring which is rotatable and axially fixed relative to the shaft. A second pressure ring is rotatable and axially movable relative to the shaft. The rings are arranged rotatable relative to each other by a limited angular amount. The opposed end faces of the pressure ring and adjusting ring are provided with circumferentially extending groove pairs to accommodate at least three rolling members. The depths of the grooves are variable in opposite directions. The pressure ring acts, via an axial pressure bearing, on the friction coupling. The adjusting ring and pressure ring may be rotated relative to each other, via a rotator assembly, to radially engage one of the rings to spread apart relative to each other via the rolling members and groove pairs. The rotator assembly of the adjusting ring includes a motor-operated worm gear or pinion.
The rotary movement is preferably initiated by a simplified rotator assembly with a gearwheel drive driven by an electric motor with a gearwheel engaging the toothed edge of the respective adjusting ring. It has been found that a pulsed electric motor may be advantageous. In consequence, hysteresis losses during the increase and decrease of the axial pressure of the friction coupling are very favorable, as a result of the simultaneous engagement and disengagement processes of two couplings. Furthermore, the pulsed current of the electric motor provides a very economical basis to electronically control two electric motors which are operated simultaneously.
The rotary assembly to actuate the spreading mechanism is preferably coupled with a brake in such a way that at the point in time when the electric motor is in a current-less condition, the brake holds the respective spreading mechanism in position. When the current is reconnected, the brake is disengaged. In an advantageous embodiment, the brake may function as an electro-magnetically operated spring brake fitted either directly at the electric motor or in the housing of the rotator assembly.